As is well-known, platinum-containing materials, i.e., materials which contain at least 80 wt. % platinum, are widely used in the manufacture of glass and glass products because of their high melting temperatures, low levels of oxidation at elevated temperatures, resistance to corrosion by glass melts, and low levels of contamination of molten glass. As is also well-known, platinum-containing materials are notoriously expensive. Accordingly, substantial reductions in capital costs can be achieved by even small reductions in the amount of platinum-containing materials used in a glass manufacturing facility.
Among the valuable characteristics of platinum-containing materials is their ability to generate heat when conducting electricity. As a result, molten glass flowing through, or held in, a platinum-containing vessel can be heated by passing electrical current between one or more locations along the length of the vessel's exterior wall. Such heating is known in the art as “direct heating,” “resistance heating,” or “direct resistance heating,” the term used herein.
A major challenge in direct resistance heating is the introduction and removal of the electric current from the vessel's wall. This is not only an electrical problem, but is also a thermal problem since the materials used to carry current to and from the wall will also conduct heat away from the wall. As a result, cold spots can be generated at the wall which can be detrimental to the quality of the finished glass, especially for glasses having strict quality requirements, such as those used to make substrates for liquid crystal displays (LCDs).
One way of introducing current into a vessel's wall is through the use of an electrically-conductive flange. Examples of such flanges can be found in U.S. Pat. Nos. 6,076,375 and 7,013,677. The present invention is concerned with flanges used to introduce current into a platinum-containing vessel wall and, in particular, with the reliability and cost of such flanges.